1. Field of the Invention
The present invention relates to an electrically conductive member such as a charging member, a transfer member and a supporting member; a unit for cleaning an image holding member; and a process cartridge and an image forming apparatus using the same.
2. Description of the Related Art
In recent years, many image forming apparatuses for forming images by an electrophotographic method often include functional rollers such as a charging member, a transfer member, and a supporting member. The functional roller used herein has a desired electrical resistivity, as well as hardness, rigidity, strength, deflection, and/or surface smoothness suitable for the application thereof.
Many of the above-mentioned currently used functional rollers have semiconductive properties, comprising a core made of stainless or iron and a layer of synthetic rubber or thermoplastic resin containing carbon, a metal filler or an ion conductive agent, and whose electrical resistivity is adjusted to about 1×105 to about 1×1010 ohm.
However, the functional roller mainly made of the synthetic rubber has the following drawbacks.
(1) The rubber of an elastic layer of the functional roller includes various components such as: residuals of a reaction initiator added to a reaction system during synthesis of a base polymer, and a by product accompanying the synthesis; a low molecular component of the base polymer; and a vulcanizer, a softener and a plasticizer added to the system during molding of a rubber roller. Many of these components easily react with the surface of a photoreceptor that is an image holding member. If the semiconductive roller is left for a long time in pressurized contact with the photoreceptor (image holding member), these components seep from the functional roller and adhere to the photoreceptor or react with the photoreceptor to reform the photoreceptor.
A solution to this problem is to form, on the functional roller surface, a barrier layer for preventing the components contained in the functional roller from seeping out. However, such a roller has a multilayered structure, resulting in increased material costs and complication of a manufacturing process, and therefore increased costs of the functional roller.
(2) In a conventional functional roller, the electrical resistivity thereof is adjusted by mechanically dispersing carbon, a metal filler, or an ion conductive agent and the like in a rubber material. Accordingly, in functional rollers having a rubber layer in which carbon is dispersed, control of the electrical resistivity thereof tends to be difficult or the electrical resistivities of the functional rollers often become uneven. Moreover, in functional rollers including the ion conductive agent, the ion conductive agent seeps out in an environment of high temperature and high humidity, and contaminates an image holding member (photoreceptor and/or intermediate transfer member).
(3) In recent years, quiet operations are also required of image forming apparatuses. A so-called “charging sound” which a charging roller generates when a high frequency AC bias is superimposed on a DC bias is an unpleasant, offensive sound, and reduction thereof has become a major technical issue.
As one method for reducing the charging sound, a method has been proposed in which a weight is put into the interior of a photoreceptor serving as an image holding member, thus preventing high frequency vibrations caused by the charging roller from propagating. However, this method requires the weight to remain fixed and a new (adhesion) process for fixing the weight inside of the photoreceptor serving as the image holding member, inevitably leading to increased costs. Moreover, as an alternative for preventing charging sound, a method has been adopted in which a foamed layer is provided on a charging roller to enable the charging member itself to absorb the vibrations. However, in this method, since the foamed layer is made of a rubber material, the aforementioned problems (1) and (2) cannot be avoided.
(4) As a measure for reducing the charging sound and preventing scraping of a photoreceptor acting as an image holding member, so-called “DC charging” has been proposed in which only a DC bias is applied to a charging roller. However, in order to realize even DC charging, the charging roller is required to have a more uniform resistivity and a smoother surface than ever. It is extremely difficult for a conventional functional roller in which a conductive agent is kneaded with and dispersed in a rubber material to form clear images with DC charging (i.e., to uniformly charge the image holding member).
(5) Moreover, for reduction in unit price of prints and photocopies (referred to as reduction in running costs), a longer life of a photoreceptor as an image holding member and various types of functional rollers have been desired.
In particular, when a high frequency AC bias is superimposed on a DC bias, discharge produced in a tiny gap between a photoreceptor serving as an image holding member and a charging roller scrapes the surface of the photoreceptor acting as the image holding member due to a so-called “etching action,” greatly influencing the life of the photoreceptor serving as the image holding member. Moreover, these various types of functional rollers have a drawback in that electrifying the functional rollers for a long period of time gradually increases the electrical resistivity of the rollers, which remains a major issue to be solved.
Meanwhile, as a cleaning unit in an image forming apparatus such as an electrophotographic copier, a cleaning blade made of an elastic material such as rubber is conventionally used. A well known structure of such a cleaning blade has one edge thereof brought into contact with the surface of an image holding member such as a photoreceptor to remove a developer, such as a toner, adhering to the surface of the image holding member.
The cleaning unit has advantages in that it has a simple structure, is inexpensive, and can efficiently remove the toner. In the cleaning unit, it is very important to bring the edge of the cleaning blade into stable contact with the image holding member surface at a uniform pressure for a long period of time.
However, fusion of the toner to the edge, adhesion of paper powder, chipping of the edge due to degradation of the blade material and the like tend to cause defective cleaning. Furthermore, in a system using a toner having a small diameter to improve image quality, adhesion of the toner to the image holding member after transfer becomes extremely high due to increase in van der Waals force. Therefore, when a cleaning blade is used in such a system, it is necessary to set the contact pressure of the blade at a high value, which often causes a frictional force between the blade and the image holding member surface to increase and the blade to warp.
As an effective cleaning method for the above-mentioned system using a toner having a small diameter, a method is known in which an auxiliary brush that rotates in contact with the image holding member is provided more upstream than a cleaning blade. In this method, the adhesion of the toner firmly adhering to the image holding member surface with van der Waals force and the like is reduced with a mechanical shear force due to rotational contact of the brush to enable the cleaning blade to easily clean the image holding member (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 1-312578).
Unlike a method using only a cleaning blade for cleaning, this method can remove the toner having a small diameter even when the contact pressure of the blade is not set to a high value.
Incidentally, a substance having a smaller diameter (mean diameter of about 1 to 50 nm) than a toner, which is referred to as an external additive, is generally added to the surface of each of toner particles in order to ensure a powder flowing property, and charging, transferring and cleaning properties. The mixing amount of the external additive depends on the specific surface area of the toner particles. Therefore, the smaller the toner particle diameter, the greater the external additive amount. Moreover, the greater the amount of toner consumed during image formation, the greater the amount of external additive reaching a cleaning zone. For example, in a full color image forming apparatus for successively developing images with four colored toners, originals are often photographic originals, and the amount of toner consumed is about ten times as many as that in the case of ordinary monochrome originals. Accordingly, the amount of the external additive consumed is also very large.
When the cleaning method using the blade or the cleaning method using the combination of the auxiliary brush and the cleaning blade is used in full color image formation, external additive particles having a very small diameter aggregate at the edge portion of the blade. The aggregating external additive particles adhere to an image holding member surface with vibration of the blade edge when the image holding member moves (a so-called “stick-slip phenomenon”), generating substantial image defects such as filming.
In order to solve such a technical problem, a conventional cleaning method using a belt-type cleaning member, namely a so-called “web,” has been well known. For example, a method has been proposed in which the belt is disposed near an image holding member surface, in which a bias having a polarity opposite to that of a toner is applied to the belt, and in which an ultrasonic vibration is applied to the image holding member.
In the method, the toner can be significantly and efficiently removed, and the toner or an external additive is not pressed against the image holding member and therefore does not adhere to the image holding member.
However, in this method, since adhesion is strong, as mentioned above, and it is difficult to completely remove a large amount of the external additive, repetition of image formation causes accumulation of the external additive on the image holding member, resulting in degradation of image quality (refer to, for example, JP-A No. 60-6977).
Moreover, a method using a belt which includes, as a part of the material therefor, fabric made of a microfiber having a diameter of 15 μm or less has been proposed. According to this method, even if a toner has a small diameter, the toner can be sufficiently removed. However, when the amount of the toner which reaches a cleaning unit is large, the belt may not function adequately (refer to, for example, JP-A No. 3-196083).
For example, in the case of a full color image forming apparatus which successively develops images with four colored toners, much photograph development is involved, thus consuming about 10 times as much toner as that in the case of ordinary monochrome documents. Therefore, the toner floods at a portion of the belt contacting an image holding member surface, and some of the toner slip through the belt or adhere to the image holding member.
Accordingly, there has been a need for an electrically conductive member which does not contaminate an image holding member and the like when brought into contact with the image holding member and the like, and can stably obtain a desired electrical resistivity, and has a long life and whose physical properties hardly change. There has also been a need for a cleaning unit which can maintain good image quality for a long period of time even when a large amount of the toner having a small diameter constantly reaches the cleaning unit, and which ensures removal of toner from an image holding member and prevents an external additive from adhering to the image holding member surface. Moreover, there has been a desire for a process cartridge and an image forming apparatus which have the electrical conductive member and/or the cleaning unit, and which are highly durable and can reduce running costs.